This invention relates to a method and apparatus for monitoring the quality of liquid such as aqueous solution, and more particularly to a method and apparatus for electrode impedance analysis suitable for the direct monitoring of impurity ions in hot pure water in the primary system of a nuclear reactor.
Liquid, e.g., water, is an influential factor for the corrosion of a construction such as a nuclear reactor. One of water quality parameters indicating the degree of influence on corrosion is the conductivity of water. It is generally known that the presence of acid, neutral or alkaline impurity ions in hot water is influential on corrosion. The conductivity of water which represents the state (dissociation) of impurity ions in hot water is measured conventionally at the room temperature after the hot water has been cooled and decompressed. However, because of different temperature dependencies of the dissociation of water, dissociation of impurities and mobility of ions, it is difficult to evaluate accurately the conductivity of hot water from the conductivity of water measured at the room temperature. On this account, the hot water oriented conductivity measuring apparatus has been an indispensable mean for the quick and accurate monitoring of the hot water conductivity. Room temperature oriented conductivity measuring apparatus available in the market employ the a.c. bridge system which measures the solution resistance from the resistance between two pieces of platinum electrodes dipped in the water under measurement with an a.c. voltage with a constant frequency around 10 kHz being applied between the electrodes, and this system is also used for the measurement of the hot water conductivity. However, the a.c. bridge system when used for the hot water conductivity measurement provides a poor repeat rate for measurement results, and resulting data has a disparity of around .+-.50% at temperatures above 150.degree. C.
The major causes of disparity in the hot water measurement results are conceivably the temperature change in the resistive component due to the oxidizing and reducing reactions taking place on the interface between the platinum electrodes and water, hereinafter referred to as polarization resistance, and the temperature change in the electrical capacitive component caused by the ion adsorption layer at the interface. As a method for strictly separating and analyzing the solution resistance component between electrodes, necessary for the analysis of the above-mentioned polarization resistance component, electrical capacitance component and conductivity, there is known the a.c. impedance analysis method. The conventional method and apparatus for the a.c. impedance analysis are disclosed in an article entitled "Identification of Electrochemical Processes by Frequency Response Analysis", Technical Report No. 004/83, pp. 30-31, published in August 1984 by Solartron Instruments.
In the a.c. impedance analysis method, the frequency response of the electrode impedance is measured by application of an a.c. voltage with a frequency range from a sufficiently low frequency to a sufficiently high frequency between electrodes dipped in the water under measurement, and the polarization resistance and solution resistance are evaluated separately from the electrode impedance at zero and infinite frequencies of the application voltage, respectively.
However, the a.c impedance analysis method when used in a practical water quality monitor takes a longer measurement time for the analysis of a low frequency region, and difficulty of quick measurement is a shortcoming of this method. Another problem is a significant increase in the capacitive and inductive impedance of the lead lines between the apparatus and electrodes relative to the electrode impedance to be measured in the analysis of a high frequency region, and this can be a cause of inferior measurement accuracy. Furthermore, the solution resistance itself is rendered highly frequency dependent in the high frequency region above 1 MHz, meaning a change in the solution resistance itself in the high frequency region, as described in publication "GENDAI DENKI KAGAKU", p. 39, by Tamura et al., Baifukan.